This invention relates to an apparatus for developing electrophotographic copying materials, the apparatus comprising a pair of feed rollers for applying a toner dispersion to the copying material, a distributor element for the toner dispersion which acts as a counter-electrode, and a pair of squeegee rollers for removing excess toner dispersion from the copying material.
It is well known that rigid metal printing plates and printing plates of flexible materials such as zinc oxide paper or plastic films may be used as copying materials. In order to achieve a uniform development of the electrostatic charge image on these printing plates, a developing apparatus has been proposed, in U.S. Pat. No. 3,999,511, in which, above the path of the copying material and between a pair of feed rollers and a pair of squeegee rollers, a distributor roller is provided which has a surface of soft, porous material and is grounded. It was found, however, that this apparatus suffered from the defect that thin white lines in relatively large black areas of the latent electrostatic charge image cannot be satisfactorily reproduced during development. In most cases, the thin white lines in the solid black areas are filled up with toner during development, which means that these lines are no longer only incompletely visible within the black area, so that the contrast effect of the toner image is poor.
In order to prevent thin white lines within black areas of an electrostatic charge image from being clogged by toner in an undesirable manner and thus increase the contrast of such thin white lines, it has been suggested, in U.S. patent application Ser. No. 658,504, now U.S. Pat. No. 4,073,266 issued Feb. 14, 1978 to use a developing apparatus in which the distributor roller for the toner has a surface of soft, porous, non-conductive material which is in contact with the copying material during its passage through the developing zone and has a metal core which is connected to the first electrode of an adjustable voltage source. The voltage applied to the distributor roller has the same sign as the surface charge on the latent electrostatic image on the copying material. In this developing apparatus, the distributor roller, which uniformly distributes the toner on the printing plate, must be lifted by the copying material, i.e. a rigid metal printing plate, as it passes through the apparatus. This provides no difficulties in the case of rigid printing plates, but is not suitable for use with thin flexible printing plates, such as zinc oxide paper or plastic film, because their stability is not sufficient to lift the distributor roller without damage to the printing plate. Moreover, breaking of the liquid developer film on the printing plate is often encountered in this apparatus during passage of the printing plate between the guide plate and the distributor roller.